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Posted by: Neil on 09/04/2020
I very recently carried out a survey on a property in which the clients had been led to believe that they had a significant rising damp problem which was affecting many of the internal structural walls on the ground floor level. When the base of the internal structural walls were tested with an electronic moisture meter, many of the walls recorded elevated moisture meter readings, but the profile of the readings was very distinct in that the elevated moisture meter readings appeared to be restricted to a height of 250 mm above the internal floor level which immediately indicated that the cause of the elevated moisture levels was not due to rising damp but some other underlying issue. In my experience, when this type of moisture profile is recorded, it is more often than not, due to the original damp proof course having been bridged by the wall plaster.
The visual evidence indicated that the wall plaster directly above the skirting board has been replaced and the distinct horizontal line which was visible at approximately 250 mm above the internal floor level provided the evidence to support my initial diagnosis that the wall plaster has bridged the damp proof course. I suggested to the client that the original skirting board (which was approximately 250 mm in height) had been removed and subsequently replaced with a smaller skirting board measuring approximately 130 mm in height which resulted in a 120 mm section of bare masonry directly above the new skirting board, so in order to deal with this, the wall plaster was extended down to the floor level using a lightweight plaster prior to the new skirting board being installed.
It is surprising how many contractors do not understand the purpose of a skirting board, the sole purpose of the skirting board is to conceal the gap between the wall plaster and the floor. I normally recommend that the wall plaster is stopped 75mm above the floor. Why 75 mm? Because a brick is approximately 75 mm in height (depending on whether it is an old imperial brick or a modern metric brick) so the damp proof course may be found up to 75 mm above the internal floor level as it will normally be in the first mortar bed above the internal floor. When I am lecturing I often ask those in attendance what the purpose of a skirting board is and the most common answer that I receive is 'that it is designed to prevent damage to the wall plaster when vacuuming the carpets' which whilst being somewhat amusing is totally incorrect.
To confirm my initial diagnosis, I did ask the client if I could remove sections of the skirting board and they were happy for me to do so. I have attached some photographs which clearly show that the wall plaster had indeed been taken down to the floor which had resulted in the bridging of the original bitumen damp proof course which on first appearances to the untrained eye or uneducated surveyor could easily have been misdiagnosed for rising damp. The obvious giveaway that the cause of the damp was not due to rising damp was the distinct horizontal moisture profile, which was recorded above the skirting boards, in my experience rising damp does not occur in horizontal straight lines.
In view of my findings, I recommended to the client that there were two options as to how this issue could be addressed. The first would be to remove the skirting boards and then cut back the wall plaster 75 mm from the internal floor level prior to refixing the skirting boards, ensuring that the face of the skirting board in contact with the wall was protected using a suitable waterproof coating or a physical barrier to prevent moisture migrating into the timber during the drying out period. There is a slight risk that 'Hygroscopic salts' may be present in the wall plaster, which may result in some damp staining as the relative humidity in the property increases and if this occurs, then the salt contaminated plaster would have to be removed and subsequently replaced with a suitable alternative but in my experience, it is far more likely that the walls will simply dry out without the need for further invasive and expensive remedial work.
The second option, if the client is not prepared to accept the minor risk detailed previously is that they should consider removing the potentially salt contaminated wall plaster and then replacing it with a suitable alternative which would eliminate the risk of the client from having to address ongoing issues at a later date.
To the more experienced surveyors and contractors, the above may seem obvious, but in my experience, this is one of the most common causes of misdiagnoses of rising damp.
I hope that you have found this article helpful but if you have any queries please feel free to get in touch.